/etc/network.subr list_net_interfaces() fix

By the time list_net_interfaces() is called in /etc/rc.d/netif,
clone_up() has
already created each of $cloned_interfaces. At this point, they will be
shown by the 'ifconfig -l', so there is no need to explicitly append
them.
In doing so, the cloned interfaces are configured and listed twice.

clone_up() is called from network_start(), before network_common() in
/etc/rc.d/netif. list_net_interfaces() is only called from
network_common()
and /etc/rc.d/dhclient, which is itself dependent on netif. Therefore,
appending the cloned interface list here will always result in
duplicates.